1. Field of the Invention
The present invention relates generally to computed tomographic (CT) imaging using a cone beam radiation source for performing an exact image reconstruction of an object, and more specifically to a method and apparatus for performing image reconstruction by individually processing multiple sub-sets of Radon data. Each of the sub-sets of Radon data is targeted for reconstructing a local 2D region-of-interest (ROI) in a 2D parallel projection of the object, and after multiple local 2D ROI's are reconstructed they are jointly processed for developing a portion of the image reconstruction of the object.
2. Description of the Background Art
Recently a system employing cone beam geometry has been developed for three-dimensional (3D) computed tomographic (CT) imaging that includes a cone beam x-ray source and a 2D area detector. An object to be imaged is scanned, preferably over a 360.degree. angular range and along its entire length, by any one of various methods wherein the position of the area detector is fixed relative to the source, and relative rotational and translational movement between the source and object provides the scanning (irradiation of the object by radiation energy). The cone beam approach for 3D CT has the potential to achieve 3D imaging in both medical and industrial applications with improved speed, as well as improved dose utilization when compared with conventional 3D CT apparatus (i.e., a stack of slices approach obtained using parallel or fan beam x-rays).
As a result of the relative movement of the cone beam source to a plurality of source positions (i.e., "views") along the scan path, the detector acquires a corresponding plurality of sets of cone beam projected measurement data (referred to hereinafter as measurement data), each set of measurement data being representative of x-ray attenuation caused by the object at a respective one of the source positions. After completion of measurement data acquisition, the measurement data is processed for reconstructing a 3D image of the object.
However, before one can perform accurate 3D imaging of the object (or a region of interest in the object), one needs a complete set of measurement data, i.e., one needs to satisfy completeness criteria. Basically, what is required is that planes, referred to hereinafter as "integration planes", that are within the field of view of the radiation source that pass through the object, or 3D region of interest of the object, and also intersect the scan path at one or more locations, develop measurement data that must be processed to accurately perform the image reconstruction. These criteria are well known, and are described in detail, for example, in U.S. Pat. No. 5,383,119 entitled METHOD AND APPARATUS FOR ACQUIRING COMPLETE RADON DATA FOR EXACTLY RECONSTRUCTING A THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY IMAGE OF A PORTION OF AN OBJECT RADIATED BY A CONE BEAM SOURCE issued on Jan. 17, 1995, hereby incorporated by reference. This patent notes that the acquired sets of measurement data are complete only if they can be processed so as to develop Radon data of sufficient density in a so-called "region of support" (a region which topologically corresponds to the field of view occupied by the region of interest of the object in real space), which can subsequently be processed to reconstruct the object with the desired resolution, and without artifacts. Typically, sufficient Radon data is acquired by exposing the entire object within the field of view to the radiation source.
Sufficient filling of the Radon space by cone beam CT apparatus having various scanning trajectories (paths) and using an area detector which is smaller than the region of interest being imaged, are known for performing an exact image reconstruction. For example, FIG. 1 herein illustrates the scanning technique disclosed in U.S. Pat. No. 5,463,666 entitled HELICAL AND CIRCLE SCAN REGION OF INTEREST COMPUTERIZED TOMOGRAPHY issued Oct. 31, 1995. A region of interest portion 10 (dark shading) of an object 12 can be imaged without blurring or artifact introduction by providing a scan path 14 consisting of a central spiral portion 16 having a circle portion 18.sub.U and 18.sub.L at upper and lower ends, respectively, of the spiral portion which are level with upper and lower boundaries of the region of interest of the object. The switch from a spiral scan path to a circular scan path is necessary in order to obtain complete cone beam data at the upper and lower boundaries of the region of interest without causing blurring caused by imaging portions of the object not within the region of interest, as described in greater detail in the fore-noted U.S. Pat. No. 5,463,666.
Although the above and other techniques have been useful, they require scan paths which have abrupt shifts in movement. Such abrupt shifts in scan movement are undesirable in that they either subject the patient to undesired jostling, or subject the imaging system equipment to extra mechanical stress. It would be desirable to only provide a smoothly changing scan path.
Furthermore, it is noted that compared with the processing required for reconstructing an image when using an x-ray source supplying parallel or fan beams, the processing of the measurement data acquired when using a cone beam source is computationally much more complex. This is because when using a parallel or fan beam source, the measurement data is already directly representative of a 2D Radon transform of a cross-section of the object. However, this is not the case when using a cone beam source, and complex processing of the acquired measurement data is required to develop appropriate Radon transform data. Such processing for exactly reconstructing an image of the object typically, comprises:
1) conversion of the measurement data to Radon derivative data. This may be accomplished using the techniques described in U.S. Pat. No. 5,257,183 entitled METHOD AND APPARATUS FOR CONVERTING CONE BEAM X-RAY PROJECTION DATA TO PLANAR INTEGRAL AND RECONSTRUCTING A THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY (CT) IMAGE OF AN OBJECT issued Oct. 26, 1993, hereby incorporated by reference, PA1 2) conversion of the Radon derivative data to Radon data at polar grid points using, for example, the technique described in U.S. Pat. No. 5,446,776 entitled TOMOGRAPHY WITH GENERATION OF RADON DATA ON POLAR GRID POINTS issued Aug. 8, 1995, also hereby incorporated by reference, and PA1 3) performing an inverse 3D Radon transformation of the Radon data using known techniques, such as those described in detail in the forenoted U.S. Pat. No. 5,257,183 for reconstructing image data that, when applied to a display, provides a view of the 3D CT image of the object.
Although the theory for exactly reconstructing an image using cone beam measurement data is generally known, such as from the US patents noted above, a practical implementation of the processing turns out to be quite problematic. Not only is the amount of measurement data to be processed very large and rapidly acquired in accordance with a timing that is mainly determined by the geometry of the scan path, but, as noted above, the calculations required on the acquired data are quite complex. The most computationally expensive part of the object reconstruction is the calculation of the Radon derivative data (steps 1 and 2 noted above). Typically one needs to calculate about 100.times.10.sup.6 line integral derivatives during object reconstruction. Since each line integral derivative requires the calculation of two single line integrals (because one uses the difference between two closely spaced line integrals to calculate a single line integral derivative) 200.times.10.sup.6 single line integral calculations are required. However, before one can even begin to perform these line integral derivative calculations, one has to compute for each Radon sample which source positions will provide the measurement data that must be processed, and determine the extent of the lines on the measurement data along which the integration must be performed. These latter determinations involve highly nonlinear calculations and are therefore computationally costly and time consuming, undesirably delaying an image reconstruction of the object.
FIG. 2 herein generally illustrates the conventional prior art technique for processing the acquired measurement data for image reconstruction. As shown, sets of measurement data MD.sub.1, MD.sub.2, MD.sub.3 . . . MD.sub.N obtained at corresponding source positions SP.sub.1, SP.sub.2, SP.sub.3 . . . SP.sub.N of the radiation source along scan path 16, are sequentially processed to develop a sufficient amount of Radon data to uniformly fill up a Radon space 200 defined by a spherical coordinate system (r, .theta., .psi.), with sufficient density to allow image reconstruction of object 16 at a desired resolution and without artifacts. The Radon data is developed at a large number of radially arranged polar grid points on a plurality of .psi.-planes 202, by conversion processor 204 operating according to the above steps 1) and 2). As previously noted, the development of the Radon data is not complete until measurement data has been obtained from all the source positions of the scan path, including those that lie on both of the top and bottom circle portions. Only after the Radon data is completely developed is it subjected to inverse transform processing according to the above step 3), via processor 206, and then passed to a display 208 for visualizing the image reconstruction.
The consequence of the above is two-fold, 1) image reconstruction is undesirably delayed until the entire 3D region-of-interest of the object is scanned by the source/detector arrangement, and 2) a very large amount of system memory must be allocated for storage of all of the calculated Radon data, since inversion processing uses the global Radon transform of the entire 3D region-of-interest of the object.
It would be desirable to reduce the time delay before which the user can begin to obtain image reconstruction, and at the same time, reduce the data storage requirements of the imaging system. Furthermore, it would be desirable that these goals be achieved in a practical manner that doesn't significantly increase the complexity or degrade the performance of the exact cone beam image reconstruction.